The use of implantable articles, such as porous synthetic vascular grafts, is a well accepted practice in the art. To improve certain properties of an implantable article, it is known to coat one or more surfaces of such articles with bio-compatible compositions. These coating compositions serve many different functions. For example, such coatings may render porous implantable articles blood-tight. In particular, U.S. Pat. No. 4,842,575 to Hoffman, Jr. et al. describes a process for rendering a synthetic vascular graft blood-tight by massaging a collagen preparation into the porous structure of the graft.
Alternatively, such coatings may be used to deliver certain pharmaceutical agents to targeted areas on an implantable article. For example, U.S. Pat. No. 5,290,271 to Jernberg describes encapsulated chemotherapeutics dispersed within a fluid or gel which are applied to a surface of an implant. In this way, the chemotherapeutic agents are released overtime to targeted areas on the implant.
Moreover, it is well known in the art to combine antibiotics, anti-thrombogenic agents and the like into coating and/or impregnating compositions that are applied to implantable articles. Such coatings increase the bio-compatibility of the implantable article by, for example, decreasing the risk of infection and blood clot formation thereon.
Coating and impregnation compositions for implantable articles like those described hereinabove can be made from a variety of materials. Such materials include, for example, biological molecule-containing compositions, polymer-containing compositions and hybrid polymer-biological molecule-containing compositions. For example, coating compositions known in the art for implantable articles include segmented linear polymers (U.S. Pat. No. 3,804,812 to Koroscil), heparinized polyurethane (U.S. Pat. No. 3,766,104 to Bonin et al.) and block copolymers of polysiloxane and polyurethane (U.S. Pat. No. 3,562,352 to Nyilas). Such compositions, however, may contain unreacted functional groups which participate in undesirable side reactions in vivo and can inhibit cell ingrowth into, for example, a vascular graft. Such complications can lead to thrombus formation, infection, etc. at the implantation site.
Biological molecule-containing coatings include, for example, such extracellular matrix proteins as collagen, fibronectin, laminin and hyaluronic acid. The use of a slurry composition containing collagen to reduce the porosity of porous textile grafts is described in U.S. Pat. Nos. 4,842,575 and 5,108,424 to Hoffman et al. both of which are hereby incorporated by reference. During the processing of such prior art slurries, collagen of appropriate size and purity was obtained from previously processed calf skins that were passed through a meat grinder and extruded through a series of filter sieves of constantly decreasing mesh size. A plasticizer was then added to the collagen slurry and the composition was applied to, e.g., the surface of a porous vascular graft. The composition was then cross-linked and dried. The use of such slurries provides an implantable article, such as a vascular graft, with acceptable bio-compatibility and blood-tightness.
Room temperature grinding of, for example, bovine hides as a step in providing an aqueous dispersion of collagen is also described in U.S. Pat. No. 4,097,234 to Sohde et al. This patent, however, also teaches that when the pH of, for example, a preparation of bovine hides or tendons is in the range where the collagen to be isolated is easily solubilized or "swelled," the collagen fibers can become nonuniform and degraded due to the heat of friction caused from violent stirring or mechanical crushing of the preparation. Thus, Sohde et al. describe mincing bovine corium and then milling it in two successive steps at about room temperature, i.e., between 20.degree. C.-25.degree. C. The resultant aqueous dispersion is claimed to have collagen fibers of 4-12 .mu.m in diameter, 2-25 mm in length and a viscosity of between 1/5 to 1/20 that of similar prior art compositions. The end products of the Sohde et al. method include nonwoven fabric, films, membranes, tubes or sheets for use as artificial blood vessels, and sutures.
The method described by Sohde et al., however, suffers from the drawback that the grinding of the bovine tendons or hides is carried out at room temperature. Grinding of these tissues at room temperature raises the temperature of the micro-environment at the grinding site and causes the collagen to denature. This produces collagen having a higher solubility both in the medium in which it is produced and in the blood stream. Such higher solubility leads to premature absorption of the coating and can cause a deleterious affect on tissue ingrowth dynamics. Thus, the healing characteristics of the device are substantially hindered. Collagen derived from such a process is clearly not desirable as a sealant for an implantable article, such as for example, a porous vascular graft due to the risk of uneven or non-uniform distribution of the collagen particles within the sealant composition. Furthermore, the premature absorption of the collagen coating can result in undesirable leakage of blood from, e.g., a sealant coated porous vascular graft.
As an alternate method for preparing implantable collagen, several references describe cryogenic grinding of collagen. For example, U.S. Pat. No. 5,256,140 to Fallick describes a method for preparing an autologous source of injectable collagen for use in leveling skin having depressions therein. In this method, the skin of a patient who is to receive the collagen composition is made brittle by cooling it to between -10.degree. F. to -100.degree. F. (-3.8.degree. C. to -37.8.degree. C.) using, for example, liquid nitrogen. The brittle skin is then crushed using a mortar and pestle or cryogenically ground using a freezer mill. This preparation is then denatured and extracted in a weak acid solution so as to obtain denatured collagen for delivery into a patient.
Similarly, U.S. Pat. No. 5,332,802 to Kelman et al. describes auto-implantable collagen for use in plastic and ophthalmic surgery. In particular, to obtain the desired collagen preparation, a sample of a patient's skin is blended or homogenized by pulverizing the skin in a frozen state, such as by freezing the skin in liquid nitrogen and grinding the frozen skin using a mortar and pestle or by way of a cryopulverization mill. Such a treatment is used to increase the solubility of the contaminates therein and to reduce the overall processing time of the preparation.
Such cryogenic methods, however, are directed to cosmetic surgery-type applications and are unsuitable for sealant compositions used in conjunction with porous implantable articles. In particular, such methods are directed to the small-scale preparation of injectable collagen. Moreover, these compositions and methods are insufficient to produce non-denatured, uniform sized collagen preparations having highly controlled viscosity ranges.
As previously stated, collagen has been widely used as a coating and impregnating composition. In particular, its use as a fluid-tight barrier for textile prostheses, such as vascular and endovascular grafts has been very successful. Processing of collagen, however, has many difficulties, due to its inherent properties. For example, to make a reproducible collagen slurry requires certain consistencies in the raw material itself, as well as, the process steps and parameters. Naturally occurring materials such as collagen, will of course have many inherent variations. In order to produce acceptable sealant compositions, these variations must be minimized. One way to do so is through controlled sourcing and processing conditions. Notwithstanding such efforts to produce reliable and consistent compositions which are able to form reproducible sealants for porous substrates, such as vascular grafts, other difficulties are present which tend to compromise the quality and/or reproducibility of such sealants. For example, it is well known that collagen denatures above a certain temperature, e.g. 37.degree. C. Once denaturization occurs, there is a loss in its natural self-aggregating properties. As a result, cross-linking is preferred or required. Additionally, grinding of raw collagen to specific particle sizes causes localized heating above its denaturization temperature. Such denaturization may go unnoticed in the early processing stages and end up in the final product. Thus, conventional grinding methods have limited usefulness due to the exposure of, e.g., collagen, to excessive heat build-up caused by the frictional grinding forces.
The prior art has also taught that cross-linking of the collagen was an important step in forming an effective sealant composition. See, for example, U.S. Pat. Nos. 4,842,575 and 5,108,424 to Hoffman et al. described hereinabove. It has recently been discovered in the course of the present invention that by eliminating the potential for denaturization and by controlling particle size, collagen compositions can be made which, under specified viscosity ranges, form reproducible, high quality sealants. The specified particle size is obtained without the concern for denaturization due to the use of cryogenic techniques as applied to the comminution process. The homogeneous particle size promotes uniformity in coating, further enhances the self-aggregating properties of the collagen and promotes the formation of a fluid-tight barrier. As a result of the present inventive processes, effective, high quality fluid-tight barriers can be obtained without cross-linking of the collagen.
In summary, all of the above-cited references generally suffer from an inability to produce highly controllable and reproducible collagen compositions. Thus, there is a need for improved bio-compatible aqueous slurry compositions and processes for forming fluid-tight barriers on implantable articles. In particular, there is a need for improved collagen compositions which contain non-denatured collagen having a uniform particle size and which have highly controllable and reproducible viscosities. The present invention is directed to meeting these and other needs.